Hyperfluorescence shows great promise for the next generation of commercially feasible blue organic light-emitting diodes (OLEDs). High-gap matrices are currently the only available approach to suppress Dexter transfer to terminal emitter triplet states to approach the required levels of efficiency and stability, which unfortunately leads to overly complex device structures from a fabrication standpoint. To eliminate the need for matrices, we introduce a molecular design strategy where ultra-narrowband blue emitters are covalently encapsulated by insulating alkylene straps. OLEDs with simple emissive layers consisting of pristine thermally activated delayed fluorescence (TADF) hosts doped with encapsulated terminal emitters exhibit negligible external quantum efficiency (EQE) drops compared to non-doped devices, enabling an unprecedented maximum EQE of 21.5%. Simultaneously, ultra-narrow electroluminescence half-widths are afforded (14–15 nm) at desirable deep blue peak wavelengths (449 and 458 nm). To explain the near 100% internal quantum efficiency (IQE) in the absence of high gap matrices, we turn to transient absorption spectroscopy. Dexter transfer to terminal emitter triplets is directly observed for the first time in a hyperfluorescent system through comparison with a non-encapsulated analogue. It is unequivocally concluded that Dexter triplet transfer from a pristine TADF sensitiser host can be substantially reduced by an encapsulated terminal emitter, opening the door to highly efficient ‘matrix-free’ blue hyperfluorescence.
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